1

Novel Rubidium lead chloride nanocrystals: Synthesis and

characterization Daniel Amgar1, Małgorzata Wierzbowska2, Vladimir Uvarov1, Vitaly Gutkin1, Lioz

Etgar1*

1The Institute of Chemistry, The Center for Nanoscience and Nanotechnology , The Casali

Center for Applied Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel, 2Institute of High Pressure Physics, Polish Academy of Sciences,

Sokołowska 29/37, 01-142 Warsaw, Poland

*E-mail: lioz.etgar@mail.huji.ac.il

Abstract

Alkali ternary lead halides were studied intensively in the past few years, especially due to

the great interest in perovskite materials. In this paper we report on novel rubidium lead

chloride nanocrystals (NCs) with the formula Rb6Pb5Cl16, which adopt a tetragonal

symmetry. The NCs were characterized and found to be active in the UV region, with a

band-gap of ~4.05 eV. The roles of the ligands, oleic acid and oleylamine, were

investigated and found to strongly affect the morphology and composition of the NCs,

through the stabilization of the facilitated crystallization of the ionic precursors. The

effective masses were observed by density functional theory (DFT) calculations, using the

dielectric function, and the the Bohr exciton radius and exciton binding energy of the NCs

were estimated. Morover the results were supported by the DFT calculations for the

electronic properties and atomic structure.

2

Introduction

Recently, the knowledge about ternary lead halides was increased rapidly with the evolving

research about perovskite compounds. Organic-Inorganic and fully-inorganic perovskite

materials were studied intensively in the last few years, mostly in the optoelectronic and

solar fields. The excellent properties of perovskites as light harvesting materials in the

visible spectrum showed a great applicative potential, both in bulk and nanometric forms.

Particularly, the enthusiasm around the fully-inorganic perovskite derived, at first, from

stability issues. Eperon et al. reported that replacing the common organic

methylammonium (MA) cation with the inorganic cesium cation highly improves the

thermal stability of the perovskite, which is a decisive factor in solar cells activity.1 The

major studies of nanometric fully-inorganic perovskite dealt with the Cs-based perovskite.

Protesescue et al. reported on fully-inorganic nanocrystals (NCs) with intriguing optical

properties, bright and sharp emissions, high photoluminescence quantum yields (PLQY),

and high absorption coefficients.2 Few works reported on tuning the composition of the

perovskite by replacing certain atoms in the perovskite formula AMX3 (A-cation, M- di-

valent metal, X- halide) in order to discover new features and improvements, including

mix cation systems.3,4,5,6 One of the possible A-cation candidates that was suggested to

replace the MA cation7,3 was the rubidium (Rb) cation, for its advantage as an oxidation-

stable mono-valent cation. The formability of Rb-based perovskite can be calculated by

the tolerance factor equation𝑡 = $%&$'[√*($,&$')]

, which determines whether a compound will

form perovskite structure or not.8 Based on the tolerance factor, it was claimed that Rb+,

only slightly smaller than Cs+, is too small in order to fit in the cubohedral cage of the A-

cation in the bulk perovskite structure3,9, according to the perovskite expected formability

range 0.813 < t < 1.107.10,11 The tolerance factors of RbPbCl3, RbPbBr3, and RbPbI3 are

0.785, 0.781, and 0.776, respectively. Apparently, the Rb cation does not fit in the

perovskite bulk structure, but on the other hand, the synthesis of nanometric Rb-based

perovskite was never been published. The surface chemistry of nanocrystals differs from

bulk due to a looser structure, thus it is conceivable that perovskite formation would occur

in the nanoscale in different conditions than bulk. For instance, CsPbI3 was investigated as

a light harvesting bulk material in solar cells and found to be very problematic for its

unstable cubic black phase and high annealing temperature.1 However, the synthesis of

3

CsPbI3 NCs is simple in which cubic CsPbI3 perovskite NCs are obtained at lower

temperatures.2 Likewise, it is very intriguing to learn how the replacement of the A-cation

will affect the properties of the perovskite crystal, optics, and activity in the nanometric

sizes. Ghanbari and co-workers reported on RbPbI3 nanostructures for photocatalysis,

synthesized by hydrothermal method, and investigated how different molar ratios of

RbI:PbI2, reaction time and temperature affect the size and shape of the product.12

Moreover, RbPbI3 nanowires were reported. The RbPbI3 nanowires were found to absorb

below 450 nm and exhibited a good photoresponsive activity, suggesting a potential in the

optoelectronic field.13 According to the literature, integrating Rb in the perovskite lattice

result in structural distortions that can lead to a different perovskite or non-perovskite

phase.14,15 Beck et al. reported for the first time and investigated the synthesis and crystal

structure of Rb6Pb5Cl16 phase.16 Rb6Pb5Cl16 was found to crystallize in the tetragonal space

group P4/mbm with a variety of different coordination polyhedra for the cations. Later,

Monzel et al.17 reported on redetermination of the phase diagram RbCl/PbCl2, which was

first reported by K. Treis18 at 1914. They stated that RbPbCl3 is unstable at room

temperature and can be stabilized only above 320 °C. The reinvestigation also revealed the

existence of the Rb6Pb5Cl16 phase, which is stable at room temperature. Furthermore,

Linaburg reported on perovskite phases of RbPbCl3, RbPbBr3, and RbPbI3 adopting a cubic

symmetry at elevated temperatures, while in room temperature a mixture of tetragonal

Rb6Pb5Cl16 and monoclinic RbPb2C5 phases is obtained.9 This reinforces the possibility to

fabricate rubidium lead halide crystals. It is important to develop more perovskite and

perovskite-like compositions and structures in order to expand their variety of applications.

As mentioned above, RbPbCl3 has the highest tolerance factor, what encouraged us to

perform a synthesis of nanometric RbPbCl3 NCs.

Herein, we report on the synthesis of novel rubidium lead chloride NCs with the formula

Rb6Pb5Cl16. This is the first report on NCs of this material, also using rubidium carbonate

(Rb2CO3) as a new precursor for obtaining this compound, other than RbCl.9,3 Physical,

structural, and optical characterizations are shown and discussed. To support the

experimental results, we performed theoretical calculations, using density functional

theory (DFT) method19, to determine the effective masses, dielectric constant, and the Bohr

radius of the NCs.

4

Results and discussion

The rubidium lead halide NCs were synthesized by a hot injection method according to the

published procedure of protesescue et al..20,2 Briefly, two precursor solutions were

prepared. The first solution contains Rb2CO3, oleic acid (OA), and 1-octadecene (ODE)

the second solution contains lead chloride (PbCl2), OA, oleylamine (OLA), tri-n-

octylphosphine (TOP), and ODE, while OA and OLA were in a volume ratio of 1:1. Both

flasks were heated to 120 °C for 1 hour under vacuum conditions for degassing.

Afterwards, the temperature was raised to 150 °C in both flasks under Argon flow. A

desired volume of Rb-oleate precursor was injected into the PbCl2 precursor solution and

the reaction was quenched with an ice bath after a few seconds. The product was

precipitated twice with isopropanol to give a white precipitate and re-dispersed with hexane

for further characterizations.

Figure 1a shows high resolution transmission electron microscope (HR-TEM) images of

the as-synthesized NCs with a square-like shape. The inset of figure 1a presents a

magnified image of the NCs, revealing some clear black spots upon the NCs (discussed

below in further details).

Figure 1b presents the absorbance spectra of the NCs measured at the day of the synthesis

and after ~4 months while stored in ambient conditions. The abosrabance peaks remained

unchanged, showing the long-term stability of these NCs. The absorbance peak is located

at ~291 nm and the onset is at ~306 nm, which corresponds to a band-gap of ~4.05 eV.

Therefore, the optical activity of the obtained NCs is in the UV spectral region. The band

gap of the bulk crystal is 3.01 eV obtained with DFT calculations and 4.13 eV with the

pseudopotential self-interaction correction (pSIC) method21. This is in a good agreement

with the absorption measurement (see figure S1 in the supporting information (SI)). The

inset in figure 1b shows a size distribution histogram, based on more than 600 NCs,

evaluating the average side length of the NCs to be 11.29 ± 0.04 nm (the size distribution

histogram was calculated from data obtained using ImageJ software). In this particles

analysis there was an assumption of square-shaped NCs.

The band-structure derived effective masses, averaged over the crystal directions at the

valence band top (VBT) and conduction band minimum (CBM), which are 0.53 and 0.19

of the electron mass (me) for holes and electrons, respectively (see figures 1c,d). These

5

numbers, together with the calculated dielectric function and its static value ε(0)/εvacc = 4.1

(figure 1e), led to the exciton Bohr radius a0= 1.584 nm and the exciton binding energy

Eb= 0.111 eV. There is a close resemblance between the obtained parameters for

Rb6Pb5Cl16 and the corresponding numbers for cesium lead halide (CsPbX3) compounds.2

Figure 1. (a) A high resolution transmission electron microscopy (HR-TEM) image of rubidium lead chloride NCs. Inset: a magnified image of a few particles. (b) Absorbance spectra of the NCs at the day of the synthesis and after four months. Inset: A size distribution histogram of the NCs measured on 647 NCs. (c) The conduction band minimum (CBM) and the electron effective masses along G-M and G-Z crystal directions. (d) The same as (c) for the valence band top (VBT). (e) The dielectric function: its real part (upper panel) and imaginary part (bottom panel), calculated along the crystal directions (red and black lines).

Furthermore, the crystal structure of the NCs was investigated via structural analysis. X-

ray diffraction (XRD) measurement was preformed and the results are presented in figure

2a. The XRD pattern corresponds to a Rb6Pb5Cl16 tetragonal phase, which is a non-

perovskite phase. This phase was discovered by Monzel et al.17 during their work on

redetermination of the phase diagram RbCl/PbCl2, as mentioned earlier. An important

insight of this work was that the perovskite phase RbPbCl3 cannot exist at room

temperature, but as a mixture of the two previously unknown phases, Rb6Pb5Cl16 and

6

RbPb2Cl5. Only with the application of heat, a phase transition can occur to get the

perovskite phase. The first phase transition occurs above 320º to a tetragonal phase of

RbPbCl3. Higher temperatures resulted in the cubic phase of RbPbCl3 perovskite. A

repeating XRD measurement of the NCs detected 9.8% of Pb2O3 phase inside the

Rb6Pb5Cl16 NCs and 90.2% of Rb6Pb5Cl16 phase (see figure S2 in the SI section). The

Pb2O3 phase can be assigned to the black spots in the Rb6Pb5Cl16 NCs shown in figure 1a.

Previous works on CsPbX3 NCs reported on black spots that appear as a response to the

electron beam during the TEM analysis.22 In this case, the black spots were integrated in

the NCs and were observed in “fresh” areas of the TEM grid, with minimal exposure to the

beam. Nonetheless, the black spots became clearer and larger with longer exposure to the

electron beam along with decomposition of the brighter parts. There is an obvious

difference in the contrasts of the grey nanocrystals and the black spots on top of them

(figure 1a). This can imply for high concentration of Pb, which have higher electron density

than the other atoms in this compound, resulting in a higher contrast.

Dark scanning transmission electron microscopy (STEM) and energy-dispersive x-ray

spectroscopy (EDS) were applied in order to confirm the composition of the NCs and the

black spots inside the NCs, as can been seen in figure 2b and figure S3. In figure S3, the

EDS spectrum detected the atoms Rb, Pb, Cl and O in the region of interest. Following

quantification analysis determined the atomic percentage of the atoms to be 17.40, 28.89,

37.62, and 16.07, respectively. The reasonable assumption is that the ratio between Rb and

Pb is ~1:1, according to the XRD, so that the remained Pb are possibly bound to the

detected O atoms to create a lead oxide phase or alternatively, metallic lead nanoparticles,

as previously reported for similar observations.23

The inset of figure 2b shows HR-TEM image of a black spot (within the NC) with the

corresponding fast Fourier transform (FFT), related to plain (111) of metallic Pb. More

areas were scanned and the results are presented in the SI section (figure S4). EDS analysis

(figure S3) indicated a higher content of Pb, reinforcing the assumption of the black Pb-

rich particles within the Rb6Pb5Cl16 NCs to be either lead oxide or metallic Pb

nanoparticles. It is predicted that the Pb-terminated surface opens the octahedral hole with

Rb+ in the center, which is a sensitive adsorption site for the oxygen atom from the oleic

acid; leading to a termination of its molecules with the H-C=O group, instead of COOH.

7

This scenario is consistent with the crystallographic data which report 9.8% of Pb2O3 phase

in the reported NCs (figure S2). The rubidium atom takes a role of a cation in the

crystal,thus its properties to bind oxygen are opposite to the electron-rich elements of the

VI-group (i.e. Se) – present for example in PbSe where the PbxOy-type structures are not

found.24

X-ray photoelectron spectroscopy (XPS) analysis (figure S5) detected the presence of Rb,

Pb, and Cl atoms with a ratio of ~1:1 between Rb and Pb, as expected. The atomic

percentage of Cl was lower than expected, as described in table S1 in the SI section. This

may occur due to evaporation of Cl2 molecules during the measurement.

In order to study the electronic propertiess of these Rb6Pb5Cl16 NCs more deeply, valence

band (VB) XPS measurement and surface photovoltage spectroscopy (SPV) were

performed. The valence band XPS measurement was applied for determination of the

valence band (figure 2c). This measurement gives information about the density and

occupancy of electronic states in the valence band of the material, while the edge of the

graph gives an evaluation of the VBT. The inset of figure 2c presents the extracted VB

value, 0.46 eV below the Fermi level. The work function of the NCs was measured using

the SPV method and found to be 4.63 eV. An overview of the band diagram of the NCs is

presented in figure 2d.

8

Figure 2. (a) Experimental (black) and simulated (red) x-ray diffraction (XRD) patterns of rubidium lead chloride NCs that show the correspondance to a tetragonal Rb6Pb5Cl16 phase. (b) A scanning transmission electron microscopy (STEM) image. Insets: high magnification of a Pb-rich particle inside a Rb6Pb5Cl16 NC with d-spacing that corresponds to plane (111) of metallic Pb and its corresponding FFT. (c) X-ray photoelectron spectroscopy (XPS) spectrum of the NCs measuring electronic states in the valence band. Inset: a magnified area of the graph near the valence band edge. (d) A schematic overview of the band diagram of the NCs.

To learn more about the influence of the ligands on the features and the crystallization

process of the Rb6Pb5Cl16 NCs, a study of the ligands’ roles was conducted. As described

earlier, the ligands ratio in the PbCl2 precursor solution was 1:1. While keeping the total

molar content of both ligands constant, mixtures of OA:OLA in a volume ratios of 1:2 and

1:3 were synthesized and studied. The use of oleic acid and oleylamine pair is well-known

in the syntheses of perovskite NCs.25 There is a dynamic equilibrium of acid/base in similar

colloidal systems.26 In this case, the oleic acid donates a proton to the oleylamine and turns

into an oleate anion, as the oleylamine ligand is protonated and turns into an

oleylamminium cation.27 Figure 3a depicts an HR-TEM image of 1:2 OA:OLA NCs in

9

which few morphologies are observed, including relatively large rectangles, long thin

nanowires (NWs) consist of small black dots, and distorted square-like phase, similar to

the NCs with 1:1 ratio (figure 1a). Upon increasing the OLA concentration to 1:3 OA:OLA,

as shown in the HR-TEM image in figure 3b, there are similar observations as the 1:2

OA:OLA NCs. In this case, the large rectangles do not appear. A synthesis with OLA alone

(without OA) was carried out as well for a complete investigation, however, a minimal

volume of OA is necessary in order to completely dissolve the PbCl2 . Therefore, the OA

has an important role in solubilizing the PbCl2.2 Based on the HR-TEM images, it seems

that an increase of OLA excess in this synthesis affects the NCs’ shape. It was previously

claimed that acid-base pair has a definite impact on the morphology and optical properties

of a similar system of perovskite NCs.26 The new morphologies can be an indication that

the binding of OLA to the surface is chemical and not physical, so variable amounts of

OLA ligands can change the growth directions of the NCs by blocking certain

crystallographic facets. However, if the concentration of OLA is increased at the expense

of the OA, it may reduce the protonation process, resulting in less protonated OLA species,

i.e., less oleylammonium cations. These conditions may increase the coordinative bonding

of the unprotonated OLA species with the Pb2+ cations on the surface, rather than the

electrostatic interaction of the oleylammonium cations with the Cl- anions in the lattice.

This can explain the elongated structures that appear in figures 3a,b. The absorbance

measurement is shown in figure 3c presenting an optical red shift, relative to the OA:OLA

1:1 case, with peak position at ~326 nm and onset at ~336 nm. The unexpected optical shift

can indicate a possible change in the chemical composition. To decipher these results, XRD

measurement was applied for the 1:2 product. Surprisingly, the XRD spectrum detected a

main orthorhombic Pb(OH)Cl phase (figure 3d). The XRD peaks perfectly matched the

standard XRD pattern of orthorhombic Pb(OH)Cl. Previously, few works reported on the

mineral Pb(OH)Cl.28,29,30,31 Chen et al. reported on Pb(OH)Cl nanotubes that absorb in the

UV region, at 258 nm and 292 nm28, which is similar to the absorbance of the obtain

nanostructures. A possible explanation to the phase transfer into Pb(OH)Cl, based on both

TEM and XRD analyses, is that higher content of unprotonated OLA ligands may interact

with larger areas on the surface of the NCs and avoid Rb-oleate species from participating

in the crystallization of the NCs, yielding in another Rb-free crystal phase of Pb(OH)Cl. In

10

addition to the Pb(OH)Cl phase, the XRD of the 1:2 product also detected minor content

of Pb2O3 and Rb6Pb5Cl16 phases. In comparison, the XRD of the 1:3 product detected only

the Pb(OH)Cl phase (figure S6). This strengthens the suggested mechanism that an increase

of OLA content enhance the formation of Pb(OH)Cl, repelling Rb-oleate species in the

growth processes. For a better insight into this issue, the interaction of the Pb-terminated

(001) surface with the oleic acid molecules were simulated. A shorter molecule than the

OA molecule was used for the simulation (see figure S7), since the COOH group is the

functional group, which attached to the NCs surface. Three plausible geometric

configurations were assumed (figure S8) and their atomic structures were optimized. Two

of these reactions end up with the oxygen atom on top of Rb from a cavity of the triangular

shape set by three Pb surface atoms. The formation energy is -2.88 eV in a case when the

acid molecule swaps the -OH termination into =O and -H, attached directly to the C atom

which lost =O (see figure S7). The -OH group can also build the surface; if it takes the

position between two Pb atoms, then the formation energy is -2.59 eV (see figure S8).

Before the reaction, no charge transfer was observed to the surface, thus the band structure

was not affected by a proximity with the acid (see figure S9). In contrast, the adsorption of

O or OH affects the band gap, as expected (see band structures in figure S10). Both cases

cause the red shift; it is about 0.4 eV for the oxygen adsorption case, and it seems to be a

multi-excitation between many impurity states for the surface reaction with -OH. However,

the oscillator strengths need to be further investigated with more advanced methods, in

order to find which optical transitions are active.

11

Figure 3. HR-TEM images of obtained NCs with volume ratio of 1:2 (a) and 1:3 (b) of oleic acid (OA) and oleylamine (OLA), respectively (OA:OLA). (c) Absorbance spectra of the NCs in both cases. (d) An XRD pattern of the NCs with 1:2 ratio of OA:OLA that corresponds to a main orthorhombic phase of Pb(OH)Cl. The spectrum of the NCs with ligands ratio 1:3 OA:OLA is similar to the one presented in (d) and can be found in the SI section, figure S6.

12

In addition, the opposite case was examined to further understand the ligands’ roles.

Accordingly, syntheses with ligands ratios of 2:1 OA:OLA and 3:1 OA:OLA (figures

4a,b) were performed. In figure 4a, the obtained NCs have a distorted shape, compared to

the 1:1 OA:OLA NCs. According to literature, the oleate ligands are effective in the control

of the size of more isotropic structures26, as seen in figure 4a, and in contrast to the case of

OLA excess. In the case of 1:3 OA:OLA ratio, as presented in figure 4b, the NCs did not

form at all. The reason for that can be insufficient amount of OLA which can severely

affect the stabilization of the forming NCs. The concentration of OLA is probably too small

in order to form free-standing NCs due to less coordinative bonding of oleylammonium

ligands to the metal cations on the surface.

Figure 4. HR-TEM images of rubidium lead chloride NCs with volume ratios of (a) 2:1 and (b) 3:1 of oleic acid (OA) and oleylamine (OLA) ligands.

Conclusions

This paper reports the synthesis of rubidium lead chloride NCs as a novel UV-active

material with intriguing features and complicated structure. HR-TEM analysis revealed

some speckled square-like shaped NCs and XRD determined the symmetry of tetragonal

Rb6Pb5Cl16 phase. Theoretical calculations of electronic and structural properties were

correlated with the experimental data. Different ligands ratios appeared to change the

morphology and crystal structure of the NCs into a main phase of Pb(OH)Cl with an optical

13

red shift. The suggested mechanism indicates that a high content of OLA relative to OA

can affect the crystallization process by insufficient oleylammonium species to stabilize

the Pb cations. In this case there is more coordinative bonding of unprotonated OLA

ligands to the Pb cations on the surface, which minimizes the interaction of Rb-oleate

species with the growing surface. The result was a main Pb(OH)Cl phase without any Rb

cations in the lattice. A work by Shu et al.29 suggests that Pb(OH)Cl material can function

as an anode for lithium-ion battaries, which can be very attractive in the nanometric scale

as well. Moreover, it can function as a precursor in the synthesis of PbSe nanocrystals.31

Future experiments will focus on synthesis modifications in order to achieve the perovskite

phase of RbPbCl3 in the nanoscale, as well as in the bulk form. The UV optical properties

of the as-synthesized NCs can be utilized for ultra violet light emitting diodes (UV-LED)

or lasing applications.

Experimental

Chemicals. Rubidium carbonate (Rb2CO3, 99%, Sigma-Aldrich), lead (II) chloride (PbCl2,

98%, Sigma-Aldrich), oleic acid (OA, 90%, Sigma-Aldrich), oleylamine (OLA, 70%,

Sigma-Aldrich), 1-octadecene (ODE, 90%, Sigma-Aldrich), trioctylphosphine (TOP, 97%,

Strem), 2-propanol (≥99.8%, Sigma-Aldrich), and hexane (not pure, Gadot) were

purchased and used as received, without any further purification.

Preparation of Rb-oleate. Rb-oleate precursor was prepared according to previous

published procedure by Protesescue et al..2 0.138 g of Rb2CO3 were mixed with 625 µL of

oleic acid (OA) and 7.5 mL of 1-octadecene (ODE) in a 100 mL 3-neck flask. The solution

was degassed for 1h under vacuum at 120 °C and then heated to 150 °C under Ar flow.

Synthesis of Rb6Pb5Cl16 NCs. The NCs were synthesized according to Protesescue et al..2

0.052 g of PbCl2 were mixed with 0.5 mL of OA, 0.5 mL of OLA, 1 mL of TOP, and 5 mL

of ODE in an additional 100 mL 3-neck flask. The mixture was degassed for 1h under

vacuum at 120 °C and then heated to 150 °C under Ar flow. The reaction was carried out

by injection of 0.4 mL of the Rb-oleate precursor solution into the PbCl2 precursor solution.

The reaction was quenched using an ice bath after a few seconds. Isopropanol was added

to the crude solution in a volume ratio of 1:1 and the NCs were centrifuged at 6000 rpm

14

for 10 min. The last stage was repeated once and then the purified NCs were dispersed in

hexane for further characterization.

Transmission Electron Microscopy (TEM). Morphology and elemental composition of

NCs were analyzed with HR (S)TEM (High Resolution Scanning Transmission Electron

Microscope) Tecnai F20 G2 (FEI Company, U.S.A.). Samples preparation was performed

as follows: 3.5 µL of the NCs dispersion were dropped on a copper grid coated with

amorphous carbon film, then the solvent was evaporated using a vacuum chamber.

Elemental analysis of NCs was done with EDAX EDS (Energy Dispersive X-Ray

Spectroscopy) when the microscope was operated in STEM mode at accelerating voltage

200 kV.

X-ray Diffraction (XRD). Powder X-ray diffraction measurements were performed on the

D8 Advance Diffractometer (Bruker AXS, Karlsruhe, Germany) with a secondary graphite

monochromator, 2° Soller slits, and 0.2 mm receiving slit. The powder samples were

placed on low background quartz sample holders .XRD patterns within the range 3-65°C.

2θ were recorded at room temperature using CuKα radiation (λ=1.5418 Å) with the

following measurement conditions: tube voltage of 40 kV, tube current of 40 mA, step-

scan mode with a step size of 0.02° 2θ, and counting time of 1 s per step.

Absorption Measurements. Absorption spectra were recorded using Jasco V- 670

spectrophotometer.

X-ray photoelectron spectroscopy (XPS). The electronic structure and chemical state of

the Rb6Pb5Cl16 NCs were characterized by the X-ray photoelectron spectroscopy (XPS).

The XPS measurements were done using a Kratos Axis Ultra X-ray photoelectron

spectrometer (Karatos Analytical Ltd., Manchester, U.K.). Spectra were acquired using an

Al Kα monochromatic radiation source (1486.7 eV) with 90° take-off angle (normal to

analyzer). The vacuum pressure in the analyzing chamber was maintained at 2×10−9 Torr.

The XPS spectra were collected with pass energy 20 eV and step 0.1 eV. The binding

energies were calibrated relative to the C 1s peak energy position as 285.0 eV. Data

analyses were done using Casa XPS (Casa Software Ltd.) and the Vision data processing

program (Kratos Analytical Ltd.).

Surface photovoltage spectroscopy (SPS). Work function (WF) measurement was

performed using the SKP5050-SPS040 system. The contact potential difference (CPD)

15

between the sample and the vibrating tip was measured by the Kelvin probe technique. The

sample was measured in a Faraday cage under air environment in three different points.

Before the measurement, the sample was stabilized with a tip for about 15 minutes. The

scan direction was from long to short wavelength. The WF was calculated according to:

𝑊𝐹123456 = 𝑊𝐹784 + 𝐶𝑃𝐷. The WF function of the tip was calibrated above the gold

stage.

Density functional theory (DFT). The pseudopotential based plane-wave code, named

Quantum ESPRESSO32, was used for the DFT calculations. We have our pSIC

implementation33 in the above package. The band structures were accurately interpolated

via the matrix elements of the Hamiltonian between the maximally-localized Wannier

functions34 obtained with the wannier90 code35. The formation energies were calculated

from the total energies of the substrates and the final atomic configuration, via the formula:

Eform = Etotal (surface+adsorbate) – Etotal (surface) – Etotal (adsorbate). The hole and electron

effective masses were estimated from the densely interpolated bands at the VBT and CBM,

respectively, via the equation =3∗ =

=ħ@

A@B CAC@

. The dielectric function was calculated with

the epsilon.x code from the Quantum ESPRESSO suite. Thus, the excitonic parameters

were obtained from the relations: 𝑎E =FGħ@HHIJKK

3∗6@ and 𝐸M =

3∗

3NH@13.61𝑒𝑉. Further details

are given in SI.36

Acknowledgement

We would like to thank to the support by the Israel ministry of science and technology

under Israel-China grant and applied research grant.

Competing financial interests

The authors decelare no competing financial interests.

16

References

1 G. E. Eperon, G. M. Paterno, R. J. Sutton, A. Zampetti, A. A. Haghighirad, F. Cacialli,

H. J. Snaith. Inorganic caesium lead iodide perovskite solar cells, J. Mater. Chem. A.

2015, 3, 19688. 2 L. Protesescue, S. Yakunin, M. I. Bodnarchuk, F. Krieg, R. Caputo, C. H. Hendon, R.

X. Yang, A. Walsh, M. V. Kovalenko. Nanocrystals of Cesium Lead Halide Perovskites

(CsPbX3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission

with Wide Color Gamut, Nano Lett. 2015, 15, 3692. 3 M. Saliba, T. Matsui, K. Domanski, J.- Y. Seo, A. Ummadisingu, S. M. Zakeeruddin, J.-

P. Correa-Baena, W. R. Tress, A. Abate, A. Hagfeldt, M. Grätzel. Incorporation of

rubidium cations into perovskite solar cells improves photovoltaic performance, Science

2016, 354 (6309), 206. 4 Z. Nie, J. Yin, H. Zhou, N. Chai, B. Chen, Y. Zhang, K. Qu, G. Shen, H. Ma, Y. Li, J.

Zhao, X. Zhang. Layered and Pb-Free Organic–Inorganic Perovskite Materials for

Ultraviolet Photoresponse: (010)-Oriented (CH3NH3)2MnCl4 Thin Film, ACS Appl.Mater.

Interfaces 2016, 8(41), 28187. 5 K. P. Marshall, M. Walker, R. I. Walton, R. A. Hatton. Enhanced stability and efficiency

in hole-transport-layer-free CsSnI3 perovskite photovoltaics, Nature Energy 2016, 16178,

1. 6 F. Hao, C. C. Stoumpos, D. H. Cao, R. P. H. Chang, M. G. Kanatzidis. Lead-free solid-

state organic–inorganic halide perovskite solar cells, Nature Photonics 2014, 8, 489. 7 D. M. Trots, S. V. Myagkota. High-temperature structural evolution of caesium and

rubidium triiodoplumbates, J. Phys. Chem. Solids 2008, 69, 2520. 8 V. M. Goldschmidt. Die Gesetze der Krystallochemie, Naturwissenschaften, 1926, 14,

477.

17

9 M. R. Linaburg, thesis. Studies of Halide Perovskites CsPbX3, RbPbX3 (X=Cl-, Br-, I-),

and Their Solid Solutions, The Ohio State University 2015. 10 G. Kieslich, S. J. Sun, A. K. Cheetham. Solid-state principles applied to organic–

inorganic perovskites: new tricks for an old dog, Chem. Sci. 2014, 5, 4712. 11 M. R. Filip, G. E. Eperon, H. J. Snaith, F. Giustino. Steric engineering of metal-halide

perovskites with tunable optical band gaps, Nat. Commun. 2014, 5, 5757. 12 M. Ghanbari, M. Sabet, M. Salavati-Niasari. Synthesis and characterization of different

morophologies of RbPbI3 nanostructures via simple hydrothermal method and

investigation of their photocatalytic activity, J. Mater. Sci: Mater Electron 2016, 27,

8826. 13 D. -H. Lim, P. Ramasamy, D. -H. Kwak, J.-S Lee. Solutionpphase synthesis of

rubidium lead iodide orthorhombic perovskite nanowires, Nanotechnology 2017, 28,

255601. 14 C. J. Howard, H. T. Stokes. Group-Theoretical Analysis of Octahedral Tilting in

Perovskites Acta Cryst. 1998, 54, 782. 15 A. M. Glazer, The classification of tilted octahedra in perovskites, Acta Crys. 1972, 28

(11), 3384. 16 H. P. Beck, M. Schramm, R. Haberkorn. Synthesis and Crystal Structure of Rb6Pb5Cl16,

Z. anorg. Allg. Chem. 1998, 624, 393. 17 H. Monzel, M. Schramm, K. Towe, H. P. Beck. Zur Neuuntersuchung des

Phasendiagramms RbCl/PbCl2, Z. Anorg. Allg. Chem. 2000, 626, 408. 18 K. Treis, Neues Jahrbuch Mineral. Geol. 1914, 37, 784. 19 W. Kohn, L. J. Sham. Self-Consistent Equations Including Exchange and Correlation

Effects, Phys. Rev. 1965, 140, 1133. 20 C. de Mello Donega, P. Liljeroth, D. Vanmaekelbergh. Physicochemical evaluation of

the hot-injection method, a synthesis route for monodisperse nanocrystals, small 2005, 12,

1152. 21 Filippetti, A.; N. A. Spaldin. Self-interaction-corrected pseudopotential scheme for

magnetic and strongly-correlated systems, Phys. Rev. B 2003, 67, 125109. 22 Q. A. Akkerman, S. G. Motti, A. R. S. Kandada, E. Mosconi, V. D’Innocenzo, G.

Bertoni, S. Marras, B. A. Kamino, L. Miranda, F. De Angelis, A. Petrozza, M. Prato, L.

18

Manna. Solution Synthesis Approach to Colloidal Cesium Lead Halide Perovskite

Nanoplatelets with Monolayer-Level Thickness Control, J. Am. Chem. Soc. 2016, 138,

1010. 23 J. A. Sichert, Y.Tong, N. Mutz, M. Vollmer, S. Fischer, K. Z. Milowska, R. García

Cortadella, B. Nickel, C. Cardenas-Daw, J. K. Stolarczyk, A. S. Urban, J. Feldmann.

Quantum Size Effect in Organometal Halide Perovskite Nanoplatelets, Nano Lett. 2015,

15, 6521. 24 W. Ma, S. L. Swisher, T. Ewers, J. Engel, V. E. Ferry, H. A. Atwater, A. P. Alivisatos.

Photovoltaic Performance of Ultrasmall PbSe Quantum Dots, ACS Nano 2011, 5(10),

8140. 25 M. A. Boles, D. Ling, T. Hyeon, D. V. Talapin. The surface science of nanocrystals,

Nature Materials 2016, 15, 141. 26 A. Pan, B. He, X. Fan, Z. Liu, J. J. Urban, A. P. Alivisatos, L. He, Y. Liu. Insight into

the Ligand-Mediated Synthesis of colloidal CsPbBr3 Perovskite Nanocrystals: The Role

of Organic Acid, Base, and Cesium, ACS Nano 2016, 10, 7943. 27 J. De Roo, M. Ibanez, P. Geiregat, G. Nedelcu, W. Walravens, J. Maes, J. C. Martins,

I. Van Driessche, M. V. Kovalenko, Z. Hens. Highly Dynamic Ligand Binding and Light

Absorption Coefficient of Cesium Lead Bromide Perovskite Nanocrystals, ACS Nano

2016, 10, 2071. 28 Y. Chen, Q.-S. Wu, R.-H. Yin, Y.-P. Ding. Facile fabrication and optical properties of

novel Pb(OH)Cl nanotubes, J. Nanopart. Res. 2007, 9, 283. 29 J. Shu, R. Ma, L. Shao, M. Shui, D. Wang, K. Wu, N. Long, Y. Ren. Hydrothermal

fabrication of lead hydroxide chloride as a novel anode material for lithium-ion batteries

Electrochimica Acta 2013, 102, 381. 30 C. F. Woensdregt, P. Hartman. Structural morphology of cotunnite, PbCl2, laurionite,

Pb(OH)Cl, and SbSI, Journal of Crystal Growth 1988, 87, 561. 31 H. Shen, J. Li, H. Shang, J. Niu, W. Xu, H. Wang, F. Guo, L. S. Li. Phosphine-Free

Synthesis from 1D Pb(OH)Cl Nanowires to 0D and 1D PbSe Nanocrystals, ACS Appl.

Mater. Interfaces 2013, 5, 10331. 32 P. Giannozzi et al., QUANTUM ESPRESSO: a modular and open-source software

project for quantum simulations of materials, J. Phys. Condens. Matter. 2009, 21, 395502.

19

33 M. Wierzbowska, J. A. Majewski. Forces and atomic relaxation in density functional

theory with the pseudopotential self-interaction correction, Phys. Rev. B 2011, 84, 245129. 34 N. Marzari, A. A. Mostofi, J. R. Yates, I. Souza, D. Vanderbilt. Maximally localized

Wannier functions: Theory and applications, Rev. Mod. Phys. 2012, 84, 1419. 35 A. A. Mostofi, Y. R. Yates, Y. S. Lee, I. Souza, D. Vanderbilt, N. Marzari. wannier90:

A tool for obtaining maximally-localised Wannier functions, Comput. Phys. Commun.

2008, 178, 685. 36 http://www.nextnano.com/nextnano3/tutorial/1Dtutorial_QW_exciton.htm